To be honest, if I were to see my collection as an investment I would sell it now. It might go up a bit more (and certain records will always keep increasing, but that's not representative for the entire market) but I can't see prices continuing to rise.
On one hand you have a new generation of collectors (usually with a beard) who are responsible for the vinyl hype of the last 10 years. But I don't see a great deal of them sticking to collecting, and even if they do, there's not a lot of people in that group that jump from new vinyl to spending top dollar for vintage pressings of metal records.
The higher prices now are caused by...well...us. Average age here is quite high (I'm probably one of the young 'uns and I'm in my mid 30's) and for many of us there is disposable income that can be spent on records. And if you're somewhat of age, you already have a lot of records. So combine those two and it's easy to see why people can drop a lot of money on one record. Why are NWOBHM rarities so expensive? An aging group of collectors with kids out of the house and mortgages paid of.
I'd put some money on the highest bidder of that Flames of hell record or Sodom shirt not being under 30 years of age. At least here in Belgium I know most of the big collectors, and only a few are younger than me. Who will keep paying top dollar in 20 years for our records?